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frazer_ramsay
October 16, 2015
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AEM Dispatcher cache

  • October 16, 2015
  • 3 replies
  • 1408 views

We are planning to deploy AEM v6.1 and looking at some lessons learned from our v5.1 deployment I am looking to do some work around our webservers and the dispatcher + cache.

What I am wondering is can multiple instances of dispatcher share the same cache?

I know it should be set to the document root of the webserver but if we run several webservers on the same physical server keeping multiple instances of the same cache data is a waste, there is no reason they could not have a common document root.

Would this work or would it just break the dispatchers caching?

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    Best answer by joerghoh

    Hi,

    If you run multiple dispatcher instances on the same machine, you can share the cache. But on the other hand, why would you run multiple instances on the same hardware (or virtualized equivalent)? You don't gain any benefit in terms of high availability if your machine goes down. It only saves you from a crashing webserver (I cannot remember that I saw this at all) or misconfiguration.

    kind regards,
    Jörg

    3 replies

    joerghoh
    Adobe Employee
    joerghohAdobe EmployeeAccepted solution
    Adobe Employee
    October 16, 2015

    Hi,

    If you run multiple dispatcher instances on the same machine, you can share the cache. But on the other hand, why would you run multiple instances on the same hardware (or virtualized equivalent)? You don't gain any benefit in terms of high availability if your machine goes down. It only saves you from a crashing webserver (I cannot remember that I saw this at all) or misconfiguration.

    kind regards,
    Jörg

    F
    frazer_ramsayAuthor
    October 16, 2015

    Hi, thanks for the reply.

    The idea around running several instances of dispatcher is to protect sites / service.

    The plan would be to have Apache & dispatcher for each website e.g.
    Apache & Dispatcher1 - brand1.com
    Apache & Dispatcher2 - brand2.com
    Apache & Dispatcher3 - brand3.com

    If we have something like a DDOS on brand1.com that instance can be taken out while we continue to serve content from the same server for the other 2 brands. The server would get busy but we can size it to cope with all Apache instances running with MaxClients.

    joerghoh
    Adobe Employee
    joerghohAdobe Employee
    Adobe Employee
    October 16, 2015

    Hi,

    Sounds like a good approach. But consider:

    * when a DDOS hits you, it's quite likely, that the network connections are completely saturated by the overwhelming amount of incoming traffic. When you shutdown the webserver for the affected domain, this incoming traffic will still hit your webservers, thus making it hard to reach the other domains as well. In many cases the webservers itself are not the limiting factor, but rather the loadbalancer or a firewall in front of it. Or the network connectivity cannot cope with the amount of traffic.
    * For this reason you need to have the DDOS protection in front your webservers, likely on the loadbalancer level or even running it in the cloud (aka CDN).
     

    Jörg

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